Structural Engineer for New House Construction Melbourne: What You Need to Know

Building a new house in Melbourne involves more structural engineering than most people realise before they start. The soil report, the footing design, the slab system, the wall framing, the roof structure, and the retaining walls on sloping blocks all require engineering input. Whether you are building a project home or a custom design, the structural engineering requirements are determined by the site, the soil, and the design, not by the builder's preference.

This post covers what structural engineering is required for new house construction in Melbourne and what to expect from the process.

Soil report and site classification

The soil report (geotechnical investigation) is the starting point for all structural engineering on a new house. It determines the site classification under AS 2870, which drives the entire footing design.

A geotechnical report for a residential site in Melbourne typically includes soil borings or test pits to around 1.5 to 3 metres depth, a description of the soil types encountered, a classification of the soil reactivity, and a recommended site class. The engineer cannot design the footing system without this information. Using the wrong site class produces a footing system that either fails (if the soil is more reactive than assumed) or costs more than necessary (if the soil is less reactive than assumed).

Footing design to AS 2870

AS 2870 (Residential Slabs and Footings) is the Australian Standard that governs footing design for Class 1 houses. It provides prescriptive solutions for standard site conditions and requires engineered design for sites that fall outside those conditions.

For reactive clay sites (Class M through E), the footing system must be designed to limit differential movement between different parts of the slab. The key design parameters are:

• Ys (soil movement): the characteristic surface ground movement for the site class (20mm for Class M, up to 75mm for Class H2)
• Hs (depth of design suction change): the depth to which seasonal soil moisture changes affect the soil behaviour
• Edge beam depth: must be sufficient to reach below the active zone of moisture change
• Internal beam depth and spacing: designed to control differential deflection across the slab
• Stiffening rib reinforcement: to resist the bending forces imposed by soil movement

Slab-on-ground vs strip footings

The two main footing systems for residential new builds in Melbourne are the waffle pod slab (a type of slab-on-ground using polystyrene void formers) and the conventional strip footing with timber or concrete flooring above.

Waffle pod slabs are dominant in Melbourne's outer suburbs and growth corridors. They sit on the ground surface with a perimeter edge beam and internal stiffening beams. They work well on flat sites with moderate to high reactivity.

Strip footings with suspended floors are more common in inner Melbourne and on sloping sites where a ground-bearing slab would require significant fill. The strip footing carries the wall load directly into the soil, and the floor is either a suspended timber floor (bearer and joist system to AS 1684) or a suspended concrete slab.

The structural engineer selects and designs the appropriate system based on the site classification, the slope, and the building configuration.

Wall and roof framing design

Wall framing design for a new house uses AS 1684 (Residential Timber Framed Construction) as the primary reference. The engineer or building designer specifies:

• Stud size and spacing for each wall type (external, internal, load bearing)
• Top plate size and configuration
• Bracing wall locations and type to resist lateral wind loads
• Connection details between wall plates and the floor and roof structures

Roof framing follows the same process: rafter size, spacing, ridge support, ceiling joist design, and the location and type of roof bracing. For roofs with complex geometry (hips, valleys, raked ceilings), the engineer may need to produce custom calculations beyond the standard span tables in AS 1684.

Wind tie-down to AS 4055

AS 4055 (Wind Loads for Housing) assigns a wind classification to the site based on location, terrain category, and topography. Melbourne suburban sites are typically classified as N1 or N2 (Wind Region A). Coastal or elevated sites may attract N3 or higher.

The wind tie-down design creates a continuous load path from roof to footing that resists the uplift force the wind exerts on the roof. Every connection in that path must be specified: rafter-to-top-plate, top-plate-to-stud, stud-to-bottom-plate, and bottom-plate-to-footing. If any connection is missing or underspecified, the load path is interrupted and the roof can lift.

Lintel and beam design over openings

Every window and door opening in a load bearing wall requires a lintel or beam to carry the loads above the opening across the gap. Lintel sizing depends on the span of the opening, the loads above (one storey or two, roof loads, floor loads), and the material (LVL, steel, or precast concrete).

AS 1684 provides standard span tables for timber lintels in common residential applications. For openings wider than the table limits, or for openings with unusual loads above (such as a large garage door in a two-storey wall), a structural engineer calculates the required lintel size rather than using the standard tables.

Retaining walls on sloping blocks

Sloping sites often require retaining walls to create level building platforms or usable yard areas. In Victoria, retaining walls above 1 metre in height generally require a building permit and structural engineering. Walls between 0.6 and 1 metre may require engineering depending on the site conditions and proximity to a boundary or footing.

Structural engineering for a retaining wall covers:

• Assessment of the retained height and soil conditions
• Selection of wall type (concrete block, poured concrete, timber sleeper, sheet piling)
• Design to resist active earth pressure and any surcharge loads (vehicles, buildings) above the wall
• Drainage specification to prevent hydrostatic pressure behind the wall
• Footing design for the wall base

Project homes vs custom homes: what the engineer does differently

Project home builders have standard engineering packages that cover their standard house designs on standard soil conditions. The structural engineer for a project home typically certifies that the standard package applies to the specific site, or designs the footing modifications required when the site is outside the standard parameters.

Custom home builds require full engineering from scratch. The structural engineer works from the architect's drawings and the soil report to design every structural element specific to the site and the building configuration. This is more work than certifying a standard package, which is reflected in the fee.

Engineering fees for new house construction

These fees include calculations, drawings, and the engineer's certification for the building permit package. They exclude the soil report, building permit fee, and construction costs.

Frequently asked questions

Does a new house always need a structural engineer?

A registered structural engineer must certify the structural drawings for any new house building permit in Victoria. For project homes on standard sites, the builder's standard engineering package may be sufficient. For custom homes or sites with reactive clay, steep slopes, or unusual conditions, engineering input is needed beyond the standard package. The building surveyor will not issue a permit without certified structural drawings.

What happens if the soil class is worse than the builder assumed?

If the soil report reveals a higher reactivity class than the builder's standard footing was designed for, the footing must be redesigned. This is common in Melbourne's outer suburbs where Class H2 soils are prevalent. The redesigned footing uses deeper and/or wider beams, additional reinforcement, and sometimes a different footing system entirely. Construction cost increases of $5,000 to $20,000 for a standard four-bedroom house are not unusual when moving from a Class M to a Class H2 footing design.

When should I engage the structural engineer for a new house build?

As soon as you have a preliminary design and a soil report in hand. The engineer needs both to assess the site class and begin the footing design. For custom homes, engaging the engineer during the architectural design stage allows structural requirements to be incorporated into the design rather than applied retrospectively. For project homes, engaging an engineer to review the builder's standard footing against the actual soil report is advisable before signing the building contract.

What is the difference between the building permit drawings and the construction drawings?

Building permit drawings (including structural drawings) are the documents submitted to the building surveyor for permit approval. They show the design intent and structural specifications. Construction drawings are the more detailed documents the builder uses on site, which may include more dimensional information and specific fixing schedules. For smaller residential projects, the permit drawings and construction drawings may be the same document set. For larger or more complex projects, the engineer may produce a separate construction set after the permit is issued.

Does the structural engineer need to inspect the building during construction?

The building surveyor sets mandatory inspection stages in the building permit, typically including a footing inspection before concrete is poured and a frame inspection before the roof is sheeted. These inspections may be carried out by the building surveyor directly, or the permit conditions may require the structural engineer to attend. Confirm the inspection requirements with the building surveyor at permit issue so the builder can plan accordingly.